1. Field of the Invention
The present invention relates to a semiconductor device that has a circuit comprising a thin film transistor (hereinafter, referred to as a TFT) and a method for manufacturing the semiconductor device. For example, the invention relates to an electronic device carrying an electro-optical device typified by a liquid crystal display panel or a light-emitting display device that has an organic light-emitting element as a part thereof.
Note that the semiconductor device in the specification indicates overall devices that can function by using semiconductor properties, and electro-optical devices, semiconductor circuits, and electronic devices are all included in the semiconductor device.
2. Description of the Related Art
Recently, a technique for manufacturing a thin film transistor (TFT) with the use of a semiconductor film (approximately several to several hundred nm in thickness) formed over a substrate with an insulating surface has been attracting attention. The thin film transistor is widely applied to electronic devices such as an IC and an electro-optical device, and in particular, has been developed quickly as a switching element for image display devices.
As a material for a crystalline semiconductor film to be used for a TFT, silicon is mainly used. As a silicon film including a crystalline structure (hereinafter, referred to as a crystalline silicon film), a film obtained in such a way that an amorphous silicon film deposited on a substrate such as glass or quartz by plasma CVD or low-pressure CVD is crystallized by a heat treatment or laser light irradiation (hereinafter, referred to as a laser treatment in the specification) has been used.
In the laser treatment, high productivity can be obtained by forming a relatively large spot shape, for example, a square spot shape several cm square or a linear spot shape of 10 cm or more in length, at an irradiated surface. In particular, when the linear spot shape is used, the whole area of an irradiated surface can be irradiated with laser by scanning only in a direction perpendicular to the longitudinal direction of the linear laser light so that the productivity is higher, as compared with a case of using spot-shaped laser light requiring scanning back and forth and from side to side.
However, in crystallization by laser light irradiation, a precipitous temperature gradient may be generated between a substrate and a semiconductor film to lower the quality of a semiconductor film obtained after the laser light irradiation.
Consequently, the applicant has proposed Patent Reference 1, Patent Reference 2, Patent Reference 3, and Patent Reference 4 mentioned below.
In Patent Reference 3, a technique of reducing strain generated by laser light irradiation in such a way that a heat treatment is performed after crystallizing a semiconductor film by using laser light is described.                (Patent Reference 1) Japanese Patent Laid-Open No. 2002-305148        (Patent Reference 2) Japanese Patent Laid-Open No. 2002-329668        (Patent Reference 3) Japanese Patent Laid-Open No. 2002-261007        (Patent Reference 4) Japanese Patent Laid-Open No. 2002-261008        
When a semiconductor film is irradiated with laser light, the semiconductor film is instantaneously melted and expand locally. In order to reduce internal stress generated by this expansion, strain is locally generated in the semiconductor film. Accordingly, a variation of the semiconductor film is caused among portions with strain and portions without strain, and a variation of the semiconductor film is caused also by a difference in extent of strain.
In addition, the semiconductor film is densified by crystallization. This densification can be confirmed by reduction in film thickness. The semiconductor film is contracted by crystallization, which contributes to generation of strain.
In a TFT, when there is strain in a semiconductor film to serve as an active layer, a potential barrier and a trap level are formed due to this strain. Accordingly, the interface state between the active layer and a gate insulating film becomes high. In addition, when there is strain in a semiconductor film to serve as an active layer, defects in operation of a TFT are caused because an electric field is not added to the semiconductor film uniformly.
Further, strain at the surface of a semiconductor film damages the flatness of a gate insulating film deposited by sputtering or CVD. Thus, insulation failure and the like is caused, which is one of causes of lowering the reliability of a TFT. As one of factors of determining field effect mobility, surface scattering is known, and the flatness at the interface between an active layer and a gate insulating film of a TFT has a great influence on the field effect mobility. The flatter the interface is, the less the field effect mobility is affected by scattering so that a higher field effect mobility can be obtained.